A chip resistor is mainly constituted by a cuboid-shaped insulating substrate, a pair of front electrodes, a pair of back electrodes, end surface electrodes, a resistive element, a protective layer, etc. The pair of front electrodes are disposed on a front surface of the insulating substrate so as to face each other with a predetermined interval therebetween. The pair of back electrodes are disposed on a back surface of the insulating substrate so as to face each other with a predetermined interval therebetween. The end surface electrodes bridge the front electrodes and the back electrodes respectively. The resistive element bridges the front electrodes paired with each other. The protective layer covers the resistive element.
Generally, such a chip resistor is produced in the following manner. That is, electrodes, resistive elements, protective layers, etc. corresponding to a number of chip resistors are formed collectively on a large-sized aggregate substrate. Then, the aggregate substrate is divided along division lines (e.g. division grooves) arranged into a latticed pattern so that the number of chip resistors can be obtained. In such a chip resistor producing process, a resistive paste is printed and sintered on one surface of the aggregate substrate to thereby form a number of resistive elements. However, due to the influence of positional displacement or blurring during the printing or temperature unevenness in a sintering furnace etc., it is difficult to avoid generation of some variation in size or film thickness among the resistive elements. For this reason, it is necessary to perform resistance value adjustment work for forming a trimming groove in each resistive element in the state of the aggregate substrate so as to set a resistance value of the resistive element at a desired one.
In the resistance value adjustment work, laser light is applied to the resistive element to form the trimming groove therein while probes are brought into contact with the pair of front electrodes bridged by the resistive element, to thereby measure the resistance value. As the trimming groove is made longer, the resistance value of the resistive element becomes higher. Therefore, as soon as the resistance value of the resistive element as a subject to be trimmed has arrived at a target resistance value (reference resistance value), application of the laser light is stopped to terminate the resistance value adjustment work.
However, the resistance value (initial resistance value) prior to formation of the trimming groove is not always lower than the reference resistance value. Due to a variation in printing conditions, sintering conditions etc. among resistive elements, the initial resistance value may be higher than the reference resistance value. In this case, the resistance value cannot be lowered by trimming. Therefore, the resistive element having the initial resistance value has to be discarded as a defective product.
To solve this problem, the following technique has been heretofore proposed, as disclosed in JP-A-61-119004 (for example, see Patent Literature 1). That is, when an initial resistance value of a resistive element is higher than a reference resistance value, another resistive paste is printed on the resistive element and sintered again to thereby lower the initial resistance value. Then, a trimming groove is formed in the resistive element in which such a two-layer structure has been arranged. Consequently, the resistance value is adjusted. As in the background-art technique, another resistive paste is separately superimposed and printed on a resistive element which has been formed on an insulating substrate, and the resistive paste is sintered to lower the initial resistance value of the resistive element. In this manner, a component which would have been discarded as a defective product can be used as a good product. Therefore, a yield rate can be improved to provide an inexpensive chip resistor. In addition, JP-A-4-250601 discloses the following technique. That is, how much higher an initial resistance value of a resistive element is than a reference resistance value is measured, and a chip resistor is sintered again under heating conditions corresponding to a result of the measurement, so that the initial resistance value can approach the reference resistance value.